1. Field of the Invention
The present invention relates, generally, to motor vehicle lighting and, more specifically, to a motor vehicle lamp having an elongated waveguide configured to conduct light in its interior, via total internal reflection occurring on border surfaces of the waveguide, along a guidance line of the waveguide predefined by the external shape of the waveguide and running inside the waveguide, wherein the waveguide has a light exit surface.
2. Description of the Related Art
Motor vehicle lamps are known in the related art and may include an elongated waveguide configured to conduct light in its interior via total internal reflection occurring on border surfaces of the waveguide, along a guidance line of the waveguide predefined by the external shape of the waveguide and running inside the waveguide, wherein the waveguide has a light exit surface. By way of example, reference is made to “rod-shaped” waveguides, wherein a rod-shaped waveguide is understood here to be a waveguide, the length of which, measured in the direction of light transportation, is a multiple of its measured dimensions transverse to the direction of light transportation, which can be referred to as the width or thickness in order to distinguish from the term “length.” The rod-shape can be straight or curved in space, as is known, for example, from annular daytime running light lamps used in connection with automotive vehicle lighting.
In addition, there are numerous motor vehicle lamps known in the related art which utilize waveguides to generate a light distribution necessary for their function. Typically, waveguides are supplied with light generated by semiconductor light sources, such as light emitting diodes or laser diodes. The light enters the rod-shaped waveguide at one or more locations and is conducted further with minimal losses due to total internal reflection occurring on walls of the waveguide. Numerous optical light emitting elements deflect a portion of the light such that light strikes a border surface at such a steep angle that the light no longer experiences total internal reflection at that point, but rather exits the waveguide and contributes accordingly to the light distribution. If a projection image is to be obtained that is homogenous to the greatest possible extent, the waveguide is designed such that the size of the light emitting element increases over the course of the wave guide as the distance to the location where the light enters increases.
Generally, it is important that the emitted light is focused to a certain extent, in particular for the functions of daytime running lights and blinkers. Government-mandated regulations frequently require light generation to conform certain light distribution parameters, in particular minimum brightness values need to be obtained in certain spatial angular ranges. For example, a light emitting diode having a flat light exit surface emits light in a half-space, such as in an angular range of −90° to +90° vertically as well as horizontally, wherein the surface norm for the light exit surface of the light emitting diode is perpendicular to the vertical axis and perpendicular to the horizontal axis. A signal light distribution conforming to government-mandated automotive regulations may only require, in contrast, certain brightness values in an angular range of −20° to +20° horizontally and −10° to +10° vertically, wherein in each case the angles are to relate to a main beam direction of the lamp parallel to the longitudinal axis of the motor vehicle. In order to concentrate the light from the light source into an angular range of this size, the cross-section profile of the waveguide lying transverse to the main beam propagation direction of the light in the waveguide may be curved in a front region, or may be designed with a radius. The front region is the region that forms the light exit surface of the waveguide. The light generated with the light emitting elements is bundled by light refraction occurring when light exits this surface.
Because of the rounded or curved light exit surface of the waveguide, which is normally accompanied by a round cross-section of the overall waveguide (in particular, circular or elliptical), certain limits to the design necessarily arise in designing the motor vehicle lamps. Thus, tradeoffs inherently arise between the optical function and the optical effect when considering design guidelines for waveguides with a circular or elliptical cross-section. Moreover, a guidance line or central fiber of the waveguide (the curve in space that the waveguide is to follow) cannot be arbitrary. A minimum radius needs to be maintained in order to ensure that light will continue to be conducted via total internal reflection. If the guidance line has a small radius, light propagated inside the waveguide tends to strike the walls at a steeper angle than when the guidance line has a larger radius, which can lead to an undesired light emission. This minimum radius relates, linearly, to the diameter of the waveguide cross-section. A thicker waveguide needs to have a guidance line having larger minimum radii. The desired bundling effect requires a certain minimum width of the waveguide. Furthermore, the width of the waveguide (or the cross-section of the waveguide, respectively) cannot be arbitrarily varied along the course of the guidance line, because the transportation of the light via internal total reflection needs to remain intact. For example, a tapering of the waveguide cross-section leads to an expansion of the beam width of the light bundle in which the light is propagated in the waveguide. As a result, the angle of incidence for the light on the border surfaces is steeper and thus, an undesired increase in the emission probability results.
If the waveguide is to produce a light projection image that is homogenous to the greatest possible extent, such that that the course of the brightness over the entire length of the light exit surface is constant to the greatest possible extent, a portion of the light at the end of the waveguide frequently remains unused. Thus, unused light then exits at the end of the waveguide, resulting in undesired bright regions. Hiding these light exit points is frequently impractical due to spatial restrictions. A deeper cut of the light emitting element into the waveguide (which would enable a stronger emission of the light in desired spatial directions) resolves this problem only to a limited extent, because the focusing function of the curved front surface is weakened thereby. Furthermore, this results in an altered light impression at this location with respect to neighboring locations. Similarly, a conical tapering of the entire waveguide profile toward the end is possible only to a limited extent because the conditions for total internal reflection of the then expanding light bundle are no longer maintained. This then results in undesired light emission.
While vehicle lamps known in the related art have generally performed well for their intended purpose, there remains a need in the art for a motor vehicle lamp which that effects concentration of light in a predefined spatial angular range and, at the same time, allows for improved design flexibility.